Staybolt



Patented Dec. 17,1946

fr osi-lcs surnom Frederick P. Huston, Fanwood, and Lawton A. Burrows,vWoodbury. NJ., and Walter E. Lawson, Wilmington, Del.; The InternationalNi York, N. Y., Burrows and said Huston assignor to ckel Company, Inc.,New a corporation of Delaware; said said Lawson assignors to E.

I. du

VPont de Nemours & Company, Wilmington, Del., a corporation of DelawareOriginal application February 2, 1943, Serial No.

Divided and this applcation Decembei' 19, 1944, -Serial No. 568,882. n@aliada `December 16,` 1942 l e claims. d(ci. ss-Lsl lThe presentinvention relates to heat exchange apparatus and, more particularly, tothe staybolts used for securing the fire sheets and Wrapped sheets inthe ilre boxes of such heat exchange apparatus as boilers.

Conventional staybolts for use in the assembly of the nre sheets andwrapper sheets of boiler lire boxes may be described as the screwed-`through ends-riveted type of construction. In fitting rigid staybolts inthis ltype of construction the nre sheets and wrapper sheets wereprovided with aligned threaded openings through which the staybolt wasscrewed so that the ends exltended beyond the cuter faces of the firesheet and wrapper sheet a distance `at least equal to two threads on thebolt. Flexible staybolts extended through the nre sheet in a similarmanner. 'I'hese extending ends were then upset so as `o form a spreadrounded head. Considerable diiliculty has been experienced with `thistype of construction due to leakage of steam and/or water between themale and female threads where the bolt passes through thesheet. Thisdifficulty has been particularly 4acute on .the lire side of the re box.y

Difllculty was experienced not only in obtaining a tight fit between thestaybolts and the re sheets but also, particularly with staybolts of4the rigid type. it had been found that numerous bolts broke in service.It has been almost universal practice to construct nre boxes withwrapper sheets of heavier gauge than the fire sheets, and it was foundthat the staybolts which brolre I in service almost always fracturedsubstantially ir tle plane of the inner surface of the wrapper s ee Wehave invented a staybolt and a method for expanding the ends thereof inthe sheets or plates of heat exchange apparatus, boilers, or .the like,which provides uid tight joints at any elevated pressure normallyencountered in such equipment, and we also have provided staybolts whichare substantially free from breakage during use.

It is an object of the present invention to provide a staybolt withmeans for insuring a fluidtight joint with a sheet through which itpasses throughout the entire areas of adjacency between said stayboltand sheet.'

Another object of the present invention is to provide a means forexpanding staybolts into fluid-tight engagement with the sheets withwhich they are usedby the use of explosive force.

A further object of the present invention is the provision of stayboltsin which concentration oi stresses in the plane of a lire sheet and/or 2wrapper sheet due to notch eiiect in the threads is largely eliminated.l

Other objects and advantages of the present invention will becomeapparent from the foli lowing detailed description of the inventiontaken view of a reduced body staybolt screwed into a.

.tapped hole in the ilre sheet, the bolt having a charge of explosivewithin a bore provided in .this end; and

Fig. e is a fragmentary sectional view showing abolt similar to that ofFig. 3 in expanded condition following detonation of the explosive'charge.

Referring now more particularly to Fig. l, reference character 2represents the found-ation ring of a iire box having an outside nre boxsheet or wrapper sheet and an inside iire box sheet or lire sheet 8. Thelire sheet and wrapper sheet are provided with aligned threaded holes` 8in which staybolts represented generally by th-e reference character Sare litted. Two such staybolts are illustrated in Fig. 1. One ofthese,designated by the reference numeral l0, is a straight body, rigid typeof stay, whilefthe other, designated by reference character I2, isarigid stay of the reduced body type.

Straight body rigid stays ordinarily are pro- I vided with threadsthroughout theiriength, The

reduced body rigid stays, on thecontraiyare threaded only at the endsand the center region of the stay has a body of reduced diameter withrespect to the threaded ends. Both types of rigid stays may be providedwith a tell-tale hole, such as that illustratedat I4, or they may behollow .throughout their entire length as illustrated at it.

,Moreoven in order to reduce the notch eect of the threads, the threadsmay be machined off, as indicated at 22, .to provide a bolt having thelowest practical value of localized stress in the plane of the innerface of the Wrapper sheet and re sheet. The machining should extend onlyslightly under the root diameter of the vthreads so as not to reduce thenet diameter or net larea of .the bolt appreciably.

The 'nre ysheet and wrapper sheet ordinarily will be made of rolledsteel plates or sheets. The staybolts may likewise be made of steel butwe prefer to make them of a nickel-copper alloy of the type soldcommercially under the trade-mark Monel" which contains approximatelytwo parts of nickel to onepart of copper.

`Great difliculty had been experienced in fitting staybolts sufficientlytightly within the holes in the nre sheet and wrapper sheet that leakagewas prevented.' Attempts were made to .prevent leakage by extendingthestays beyond the plate not less than two threads when installed andthereamasar after upsetting the end to provide a rounded head f such asthat illustrated in Fig. 1 by reference character 24. This expedient didnot solve the dimculty since leakage still occurred lnboilers having thestaybolts headed in this manner.

Moreover, in certain areas of the fire box, the rounded heads of thestaybolts were rapidly cut away by the flow of cinders until the boltwas lsubstantially flush with the exposed face of the e re sheet, thusdestroying the sealing eiect that the rounded heads were designedtoproduce.

l We have discovered I'that completely uidf tight joints may be easilyand inexpensively pro- 4 l velocity explosive extending slightly beyondboth edges ofthe fire wall. A compressed primer other Words, one thatpossesses a normal velocity of detonation above 1000 meters per secondwhen shot under the usual conditions of determining velocity. Theprimary detonating compounds I may be employed, for example,mercury-fulminate, lead azide, diazodinitrophenol. Preferably,

y however, weuse explosives such at tetryl, nitroduced by expanding theends of the 'staybolts through the use of explosive charges. For thispurpose the end of the staybolt which is to be expanded is provided witha bore 26 adapted to receive an explosive charge. This bore shouldextend at least to the plane of the inner face of the re sheet orwrapper sheetand preferably extends at least about '/s to 1% .inchbeyond. In straight body staybolts the bore 26 may bea part ofthe-tell-tale hole, but in reduced body staybolts the bore 26 has alarger diameter than the tell-tale hole. The bore 26 must not extendbeyond the change of section since the net area of the cross section ofthe metal at the end should not be less than the net area in the reducedsection. The staybolt -is screwed into the threaded holes in the rlreand/or wrapper sheet in the usual manner and it may extend onlyiiushwith the exposed face as shown at 21 in Fig. 1, or it may extendany-desired distance beyond, as illusvtrated in Figs. 2, 3 and 4. InFigs. 2 and 3 the play between the bolt S and the sheet 6 prior toexpanding the end of the bolt has been exag-v gerated forpurposes ofillustration.

Either before or after the bolt is in position,

an explosive charge 28 is placed inthe bore 26.

'.Ehe explosive charge 28 may be loaded directly into the bore hole, ifdesired, as illustrated in Fig. 3. In such case it` may be desirable toinsert a disc'or closing device at the base of the hole to keep theexplosive from extending into the narrow tell-tale hole. Sincecontrolled compression of the charge is advantageous, it will ordinarilybe preferred to precharge the explosive into a cylindrical containeradapted to f it into the hole. This will obviate the hazards present inhandling and pressing exposed explosive charges under shop conditions.Various methods of bringing about the explosion are suitable, such asthe application of heat, percussion, the name from a fuse or othersource, electrical methods, etc.

mannite, pentaerythritol tetranitrate, and the like, which detonate athigh velocity and at the same time develop great' strength because ofthe larger gas volumes. The amount of explosive may depend on thediameter of bolt, the characl 'teristics of the metal of the'bolt andother factors. It will be understood that the diameter of the bore holefor containing the explosive should lbe suinciently large to accommodatea charge of the proper amount. At the same time, the thick-- ness ofmetal betweeny the center -hole and the outer bolt wall must be enoughto give.the neeessary strength when the metal has vbeen expand-v ed. Anadvantage of expanding. the staybolt in accordance with our inventionover mechanical expanding methods'lies in the uniformity of expansion inall directions. 'l

. The following examples illustrate specific embodiments of steybqltsJoined to metal wens where the ends of said .bolts have been expanded bymeans of explosive charges. 1

Example No. 1 Y

' A threaded bolt of inch diameter made of@ inserted a cylindrical metalshell, adapted lto ilt said bore and containing a compressed charge of10 grains of pentaerythritol tetranitrate at `its base, a primer chargeof 2 grains of compressed lead azide adjacent to the base charge, and anl' ignition charge of a suitable mixture. The base In Fig. 2, anassembly is shown similar to that of Fig. 3, with the preferred methodof loading and firing illustrated more'in detail. After the bolt S isscrewed into the re wall 6 so that the former extends slightly beyondthe outer edge of said wall, ametal shell 30 is inserted into the borehole 26 in the bolt and contains a compressed base charge 28 Vof a highstrength, high charge had been loaded under a pressure of about 5000pounds per square inch and extended slightly beyond both faces of thewall. The charges were brought to explosion by means of a fuse. Aclosefitting junction, impervious to highpressures, resulted from theexpansion of thebolt'metal.

Example No. 2

Similarly, a threaded 7A; inch metal bolt was screwed into another V2inch steel plate. The

bore hole in the bolt end wasof inch depth and 0.235 inch in diameter. Acylindrical metal container was inserted into thebore hole, saidcontainer being loaded with a. 10 grain base charge of tetryl, andsuitable primer'and ignition com positions. Again the base charge wassuch that explosion was brought about by fuse, and a tight jointresulted between the staybolt and the surrounding metal.

Example No. 3

Additional joints were made between staybolts and metal plates in whichthe tightness ofthe junctures were tested. A length of 4" diameter extraheavy steel pipe was first taken and closed at -both ends by weldingthereon pieces of inch boiler plate. A tube was likewise welded onto thepipe-to allow the application ofhydraulic pressure and its measurementby a gauge. Two diiierent bolts made of the` above mentionednickel-copper alloy were used with such a pressure container, beingscrewed into opposite openings in the two plates. Longitudinal holeswere drilled into the bolts, 0.235 inch in diameter to depths varyingfrom 1% inch to 1/2 inch. Charges of pentaerythritol tetranitrate wereused, varying from 5.75 grains to 9.0 grains. In some cases theexplosive charge was co-extensive with the thickness of the plate, whilein others it extended slightly beyond the edge or just to said edge. It

vwas found, generally, that the tightest joints resulted when theexplosive charge overlapped both edges of the plate slightly. In thesecases it was found that the metal of the vstaybolt surrounding the borenot only had expanded tightly into contact with the wall of theperforation in the sheet but that it had even bulged slightly on eitherside of the sheet, resulting in avery strong pressure-tight joint.Somewhat weaker joints were obtained when the explosive charge did notextend past the edge although these joints were also satisfactorilypressure-tight. Under conditions where the charge extended slightlybeyond both edges of the plate, tests indicated that no leaking occurredeven under a pressure of 4800 pounds per square inch. In no case dideven a slight leak occur at a pressure less than 2200 pounds per squareinch. This appears very striking and significant when it is realizedthat steam boilers ordinarily operate at pressures around about 225pounds to about 250 pounds per square inch.

Using a similar test and an explosive charge of 10.5 grains ofpentaerythritol tetranitrate, an unthreaded nickel-copper alloy bolt wasused. After the explosive charge had been detonated, the joint was foundto withstand a pressure cf 3200 pounds per square inch without anyleakiness, which was the maximum pressure that could be exerted on this.particular assembly.

An additional test of an annealed 18-8 chromium-nickel stainless steelbolt under similar conditions showed that leakiness occurred in no caseat a pressure below 4600 pounds per square inch. v

In the immediately foregoing examples, bolts have been used consistingof stainless steel and Vthe nickel-copper alloy sold under thetrade-mark Monel. It will be understood that various other metals havingsatisfactory mechanical, metallurgical and chemical propertiesv forstaybolts may be fitted bythe process embodying the present invention.Such metals as aluminum, nickel and copper; various nickel, aluminum andcopper alloys; steel and certain steel alloys; others may be applicable.It is preferable that a metal for expansion in accordance with ourinvention have a potential elongation value of at least 20%. A

and many it extended slightly beyond both wall edges. 'Ihe narilyoperate at a pressure between 6 Y Example No. 4

Heat exchange apparatus was constructed by means of explosive staybolts.A cylindrical catalyst chamber was spaced within a cylindricalv `liuidbeing treated in the catalyst chamber. The

connections between the bolts and the walls were effected by formingbore holes 0.235 inch in diameter in each end ofthe bolt, and about 5/8inch in depth. The metal defining these-holes was expanded into contactwith the walls to be spaced apart, by means of .an explosive capsuleloaded with a 10 grain base charge of tetryl and conventional primer andignition compositions. Explosion was brought about by means of a fuse tocause the pressure-tight joints already described.

It is not essential in all cases that the staybolts and perforations bethreaded as pressuretight joints can be produced-by the process of thepresent invention between unthreaded bolts and smooth walledlperforations or between staybolts and perforations'which are threadedforonly a portion of the thickness of thepllate. Nor

is it essential that the base chargeof the explosive extend beyond bothfaces of the re or wrapper sheet. Satisfactory fluid-tight joints arealso obtained when the bolt terminates at the plane of the exposed faceof the sheet,` as illustrated at 2l in Fig. 1. This has the advantage ofproviding a smooth surface which is not cut away to any appreciableextent by the now of the cinders therealong. The bolt may also exl tendbeyond the face of the're sheet, as shown in Figs. 2, 3, and 4 and beupset after expansion to provide a, rounded head 26 as illustrated instaybolt I2 of Fig. 1.

The present invention provides an extremely simple and rapid method oftting staybolts 'so tightly in the boiler plates that the joints arefluid tight at pressures 1000% to more than 2000% greater than theordinary working steam pressure. In the United States the boilersordithe ranges of about 200 to 300 lbs. per square inch, with an averageof about 225 to about 250 lbs. per square inch. The present inventionprovides a high safety factor even for boilers operating at very muchhigher working pressures such as have been contemplated and to a certainextent used abroad with Working pressures in excess of 1000 lbs. persquare inch. These joints are not only fluid-tight initially but theyice.` As those skilled in the art are aware, staybolts are exposed inservice to tending not only to loosen the joint between the stayboltsand the boiler plates but even to fracture the bolt. Many boilersoperate at temperatures up to 550 F. and even higher. These high temperatures impose stresses on the staybolt and the joint due to thermalexpansions of the metals of the staybolts and boiler plates. Moreover,at least once each month the locomotive must be washed out by emptyingthe hot water and relling with fresh water which sometimes is cold. Thisinvariably results in uneven cooling of the boiler producing very highstresses known in the art as wash-out stresses. The staybolts and boll-The bolts were expanded exploslvely will remain fluid-tight in serv--severe stresses I er plates are also under static stress due to the lsteam pressure. Moreover, particularly in boilers of locomotives andmarine vessels, the stay.

bolts also are exposed to dynamic stresses, mechanical shocks, andvibrations resulting from the movement of the boiler. The'movement ofthe boiler may also impose sheer stresses from lwarping or wrenchingoi.' the boiler assembly.

Thus, in locomotive boilers the weaving and warping of the locomotiveframe is transmitted to the boiler assembly and causes it also to weaveand warp. The Association of American Railroads and various engineeringsocieties, including the American Society of Mechanical Engineers andthe American-Society for Testing Materials,

as a result of many' years of experience and re' search, have developedcodes and specications,

-deslgned to reduce staybolt breakage and leakage to a minimum. See forexample, the A. S.

Boiler Construction Code, 1940 edition, and the specifications lof the.Association ofy American Railroads, 1939 revision.` Some of-:tlie moreimcontribution to the boiler art in greatly reducing such breakage andleakage. Thev ends of the bolts that are explosively expanded by themethodv embodying the present invention contact the."wa11s of the holesin the boiler sheets so tightly that the joints will remain fluid tightin service despite thermal stresses due to heating, cooling, wash-outs,etc., and mechanical stresses resulting from shocks, vibration, warping,and the like.

The present application is a division of our copending applicationSerial No. 474,480, led Feb ruary 2, `1943. l y

Although the invention has been described and illustrated in connectionwith certain-specic embodiments, it will be appreciated that manymodiiications maybe made therein without departing from the scope of theinvention as deilnedin portant provisions relating to stayboltsappearing in these codes and specifications are as follows:

,Physical and mechanical properties v Tensile strength: 47,000 lbs. persq. inch minimum Yield point 60% oil the tensile strength as a. min- YElongatlon in 8 inches: 28% minimum.

Reduction in area: 48% minimum.

Maximum allowable stress on staybolts: 7,500 lbs.

per sq.,inch and 8,000 lbs. per square inch', depending upon thestaybolt length, design and construction.

Permissible variation in size: The staybolt must be truly round within0.01 inch and the diameter shall not be less than 0.005 inch over normtcre than 0.020 inch over the specified diame r.

month and after every hydrostatic test.

The hydrostaticy test shall be made at least every twelve months and itmust `be carried out at greater pressure than the working steam thestaybolts iitted in accordance with the present invention will standover pressures from 8 to i6 times that specified for the hydrostatictest.

Even vwith the precautions taken in these speciilcations and codesbreakageof the staybolt and leakage `between the staybolts and theboiler the following claims.

We claim: j 1. A metallic staybolt having circular ends adapted forinsertion into circular holes in re- 'spective plates, whereby saidplates are to. be

joined and maintained at a predetermined distance apart, bores providedin each of said ends Tests: steybeits shan be tested et least every ipressure. In this'connectionit is signicant that plates have occurredinnormal operation of boil ers. The' present invention makes an importantsule containing a charge of high velocity explosive substantiallyillling at 'least one of said bores, said bores being open for fescapeof the products of explosion, and means for detonating said charge.

`3. A metallic staybolt having circular threaded ends adapted forinsertion into circular threaded holes vin respective plates, wherebysaid plates are to be joined and maintained at a predetermined distanceapart, each of said ends beingprovided with a concentric bore ofsuiiicient depth to extend at least to the inner face of the respectiveplate, a capsule containing a charge of high velocity explosivesubstantially filling each of said bores, said bores being open forescape oi the products of explosion, and means for detonating saidcharges. i

- FREDERICK P. HUSTON.

LAWTON A. BURROWS.

WALTER E; LAWSON.

